1. Field of the Invention
The present invention relates to a recording medium lift detection apparatus and an inkjet recording apparatus. Particularly, the present invention relates to a recording medium lift detection apparatus and an inkjet recording apparatus capable of preventing erroneous detection of a lift of a recording medium.
2. Description of the Related Art
An inkjet recording apparatus ejects liquid droplets from an inkjet head to record an image on a recording surface of a recording medium such as a sheet conveyed through a predetermined conveyance path. When the conveyed recording medium is lifted off a conveyance surface in such an inkjet recording apparatus, the distance (slow distance) between a liquid droplet ejection surface of the inkjet head and the recording surface of the recording medium changes, degrading the recording quality level, or damaging the liquid droplet ejection surface as the recording medium comes into contact with the liquid droplet ejection surface of the head.
Therefore, a recording medium lift detection apparatus is provided in the conveyance path through which the recording medium is conveyed, in order to stop the conveyance of the recording medium when a detected lift of the recording medium is at least a specified value. For example, Japanese Patent Application Publication No. 2010-76872 describes a recording medium lift detection apparatus which is provided with a light projection part and a light receiving part facing each other with a recording medium conveyance path therebetween, wherein the light projection part emits a detection beam toward the light receiving part at a predetermined height position from the conveyance surface and whether the detection beam is received or not is detected.
However, the recording medium lift detection apparatus of Japanese Patent Application Publication No. 2010-76872 which detects the lift of the recording medium based on changes in the amount of light received by the light receiving part when the recording medium blocks the detection beam, often detects erroneously that the recording medium is lifted even when it is not lifted.
The inventors of the present invention had performed keen investigations and found out that such erroneous detection is caused by the change of the amount of light received by the light receiving part. This change of the received amount of light is caused by the fact that air of different temperatures or humidity, or air of different densities, are produced by various devices provided in the inkjet recording apparatus, and this air of different densities flows into the optical path of the detection beam and the detection beam is refracted.
Japanese Patent Application Publication No. 2008-126155 describes an invention that covers an optical axis with an elongated optical axis cover member in order to eliminate such influence of the air of different densities when the detection beam is refracted due to the air of different densities and consequently the optical path of the detection beam is bent as described above.
However, the method of covering the optical axis with the elongated optical axis cover member as described in Japanese Patent Application Publication No. 2008-126155 might cause a recording medium to come into contact with the optical axis cover member when the recording medium is lifted, thereby damaging the recording medium.
Further, even if the recording medium is lifted, the optical axis cover member prevents the recording medium from blocking the optical path, so that the lift of the recording medium cannot be detected. In addition, when adjusting the height of the optical axis in accordance with the thickness of the recording medium, the height of the optical axis cover member needs to be adjusted, which is troublesome. In order to accomplish such a task, a device for changing the height of the optical axis cover member is needed.